Variable induction coil



A 2, 1949. G, GUANELLA 2,477,693

VARIABLE INDUCTION COIL Flled Nov. 2, 1946 2 Sheets-Sheet l JNVENTOR. GY/SMV Z/A/VELLA BY m4 2, 1949- G. GUANELLA 2,477,693

VARIABLE INDUCTION COIL Flled NOV. 2, 9 5 2 Sheets--Shee t 2 17 T 7 JNVENTOR. 6'UJ7J4V GUAM/ELLA Patented Aug. 2, 1949 VARIABLE INDUCTION COIL Gustav Guanella, Zurich, Switzerland, assignor to Radio Patents Corporation, New York, N. Y., a corporation of'New York Application November 2, 1946, Serial No. 707,512 In Switzerland December 17, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires December 17, 1962 6 Claims.

This application is a continuation-in-part of my application Ser. No. 516,265, filed December 30, 1943, now abandoned, entitled High frequency coil with variable inductance.

The present invention relates to induction coils with adjustable inductance, more particularly to a variable induction coil structure for use in connection with a condenser to provide a tuning or resonant circuit of variable resonant frequency.

According to known methods of varying the inductance of a coil, the effective inductance of the coil is controlled by means of a slidin contact moving either spirally along the individual coil turns or in the direction of the coil axis. A disadvantage of arrangements of this type is the fact that the contact with the wire is highly inefficient and subject to considerable wear during use, and that furthermore the free or open end of the coil constitutes an undesirable oscillating system causing interference and other defects well known.

It has also been proposed heretofore to vary the inductance by means of a non-magnetic metal ring or cylinder arranged to displace the magnetic flux of an induction coil, said ring or cylinder being adjustable in an axial direction and being arranged either outside or inside said coil. In inductance coils of this latter type considerable eddy current losses are produced, while the adjusting range of the inductance is relatively limited.

According to the present invention, the above disadvantages are substantially avoided by the provision of an adjustable non-magnetic metal of this type member associated and cooperating with the coil windings in a novel manner, whereby the alternating current through the coil is passed at least in part through said metal member by way of the inherent electrical capacity between said member and the adjacent coil winding turns.

The invention, both as to its further objects and novel aspects, will become more apparent from the following detailed description of a few practical embodiments and applications, taken in reference to the accompanying drawings forming part of this specification and wherein:

Fig. 1 shows schematically the basic construction of a variable induction coil embodying the principles of the invention;

Fig. 2 shows an equivalent electrical circuit diagram explanatory of the operation and function of Fig. 1;

Fig. 3 illustrates a modification of Fig. 1;

Fig. 4 shows an equivalent circuit diagram for the construction according to Fig. 3;

Fig. 5 and Fig. 6 are circuit diagrams illustrating, by way of example, the employment of an induction coil according to the invention in a push-and-pull vacuum tube circuit;

Fig. 7 shows an improved adjustable induction coil according to the invention designed to increase the accuracy of the inductance control;

Fig. 8 shows still another modification of a variable induction coil structure embodying the principles of the invention, and

Figs. 9, 10 and 11 are circuit diagrams illustrating further practical applications of the invention.

Like reference characters identify like parts throughout the different views of the drawings.

Referrin more particularly to Fig. l, the induction coil shown at 10 may be of any suitable construction, that is either of the self-supporting type or wound upon a suitable support in a manner well known. A non-magnetic hollow metallic cylinder H concentrically surrounding coil [0 is arranged to be displaced or shifted in an axial direction. For this purpose, the cylinder H is shown provided with a pair of spaced annular flanges 42 and I3 which are in turn provided with openings at their upper ends through which is passed a fixed guide rod 14. Additional threaded openings at the lower ends of said flanges cooperate with a threaded spindle carrying an adjustin knob I6 at its extreme end. Thus, by turning knob IS, the cylinder 4 I ma be displaced axially with respect to the coil l0, whereby to cause the ratio of the free or open coil portion A-B to the covered or enclosed coil portion B-C to vary in a manner well understood. Any other device or mechanism for adjusting the cylinder ll may be employed for the purpose of the invention.

If the distance of the winding turns of coil Hi from the inner surface of the cylinder 1 l is small compared with the distance between adjacent winding turns, the mutual inductance between the individual winding turns inside the cylinder will be practically negligible. The enclosed portion of the coil between points 13 and C covered by the cylinder ll may therefore be replaced by an equivalent straight conductor having a length equal to the stretched or unwound length of the enclosed coil portion BC and being spaced from a conducting surface by a distance equal to the spacing of coil I 0 from the cylinder II. In other words, coil portion B-C a flexible conductor as shownfa'slidingcontact.

arrangement, or in any other suitable'manner, Cylinder I I is in turn connected at its inner point E, to the electrical circuit in which the'coil is to be inserted, i. e. terminal I), the remaining terminal a of the coil being connectedtq the beginning of the coil winding at point A. In; the,

example shown, points A and E are furthermore connected to a fixed condenser II, the connection to point E being through. a flexible conductor or the like, t provide an adjustable parallel-resonant or tuning circuit, whose terminalsa and b. may be connected in any desired system or circuit such. as an amplifier, oscillator or the like. Alternatively, the coil terminals A and Emay be connected to the condenser I! in series to obtain a series-resonant circuit in accordance with well known practice.

In Fig. 2 there isshown an equivalent electrical circuit diagram of thecoil structure described hereinbefore. The inductance of the open or effective coil portion identified by numeral I; is'in series with the Lecher linev luff-I I formed by the adjustable. cylinder corresponding to conductor II and the adjacent ineffective coil winding turns corresponding to conductor I ll' the remaining reference characters having the same significance as those of Fig.1. In this modification of'the invention, the end C of coil Ill-inside the cylinder II is directly-connected to'the latter as indicated by the. direct or conductive connection between points'C and-D. The'Lecher line- I0-I I thus acts mainly as a simple capacity which, compared with the inductance of 'the' effective coil portion I0, mayben'eglected for'all practical purposes.

In place of a metallic contact between thecoil I0 and thecylinder II, it is possible to provide a capacitative connection such as in the form of a cylindrical'condenser, by connectingtheend C'of the coil to a fixed solid cylinder I8 arranged inside the hollow sliding cylinder I'I, as more clearly shown in Fig. 3. In the latter, the axial adjustment of the cylinder II has been indicated merely schematically by the double arrow at. The adjustment may be effected by a mechanisin similarto that shown in Fig. l or, by any other" suitable means as will suggest itself to those skilled in the art.

Fig. 4 shows the equivalent circuit diagram of theconstruction shown in Fig. 3 which differs from Fig. 2 merely by the capacity IBbrid'ging the ends C and D of the Lecher line III I I.

In many cases the conductive or capacitative connection between the coil II! and slidingcyli'n der, II may be omitted, whereby theequivalent diagram of the enclosed coil portion. represents an open. Lecherjline which, if of short length, constitutes a pure capacity for all practical pur'- poses. This capacity between the, equivalent. Lecher wire conductors 'IIJ -.-I I';, Fig. 4 mayalso. be used as a circuit element passing the-electric current of the circuit in which the coil; is co ec e Shown in t e mbod men o vent'ion described in the following.

There is thus provided by the invention a noriresonant induction coil of adjustable inductance devoid of variable contacts or the like and which may serve as an efiective. tuning reactance in a resonant circuit comprising. said coil and a condenser connected therewith either in parallel or, series. In order to prevent undesirable losses due to radiation, the coil is designed as small as possible in relation to the Wave length of the highfrequency current passing through it in operation. In other words, the natural resonant frequency of the coil due to its distributed capacity should be substantially higher than the resonant frequency of the resonant circuit of which the coil forms an effective tuning element.

According. to one embodiment of the invention, the adjustable induction coil described may be advantageously used in connection with a pushand-pull vacuum tube circuit, as shown in Fig. 5. In the latter, the anodes of thetwo push-andpull connected amplifier tubes 2! andZI are'connected in a knownv mannerto a tuned circuit comprising the condenser ll shunted by two variable induction coils 22 and 23 which are constructed in accordance with the invention'and connected in series With each other. The-series connection of coils 22-and- 23 is efiected, inthe example illustrated, through the Lecher line capacities between the ineffective or covered-portions of the coils and their associated sliding cylinders 2d'and'25, respectively, said cylindersbeing both electrically and meohanioallyeonnected through a member 25 for uni-control or; synchronized adjustment, as in dica ted by' the arrows :c. If several circuits or amplifying stages are to be tuned simultaneously, it is advantageous to control all of the circuits in'synchronism by directly mechanically connecting all the tuning cylinders, inasmuch as all stages, if; designed equally will have the same frequency variation.

characteristic. Small deviations from. the synchronized operation may be corrected by the. provision of additional or trimmer condensers or by any other suitable means known in the art.

According to the arrangement shown in Fig,.

6; two coils according to the invention are employed 'for effecting an exact electrical symmetry in a symmetrical push-and-pull circuit, by displacing a common sliding cylinder 28 relative to, the symmetry point of the circuit and in the, direction of thecommon axis of the coils 26 and 27, as indicated by the arrow :13.

In order to effect a clear separation between the ineffective or enclosecl'and the eifective. or

free coil portion, to improve the accuracy of the inductance control, the sliding cylinder may be constructed in the manner shown {more clearly in Fig. 7. In the latter, the 'cylinder lI is ar ranged toperform a screw-lilie or spiral movement, with the pitch of the spiral movement sub; stantially corresponding to the pitch of the coil winding turns. Accordingly; the can windings will always be exactly underneath the helical shapedprojecting nose Not the cylinder llwhich' is out ofi perpendicularly to th'ecoil winding turns...

According. tov a further modificationof theina vention, the sliding. cylinder may .be arranged on the insideinstead of on. theoutside-ofthe c0ilas shown in the preceding. illustrations. Such, anarrangement is shown in,Fig. ,8. -.In.the latter, thesliding cylinder]! which in-this. case may; be either a hollow or a solid cylinder is arranged;- fOr i l Qll :-H Sid.:t 16 coil Him Therewis shown for this purpose a cup-shaped guiding member 31 or hollow cylinder and a piston-like member 32 sliding therein and adjustable by means of a threaded. spindle 33 suitable journalled in the end wall of member 3i and carry ing an adjusting or operating knob 34. The piston 32 is normally urged into engagement with the inner end of spindle 33 by a spring 35 in such a manner as to aiford a continuous axial adjustment of the cylinder it by rotation of the knob 34. Any other mechanism or device for axially adjusting the cylinder iI within the coil Il) may be employed for the purpose of the invention.

According to another modification of the in vention, the capacity between the coil and the sliding cylinder may be increased by using an adjustable member consisting of two cylinders, one arranged inside and the other being arranged outside the coil. If the cylinder is given a screw-like or spiral motion, the effective capacity may be further increased if the cylinder surface nearest the coil is shaped so as to conform with the profile of the coil winding turns. The capacity between the sliding cylinder and the coil may also be increased by using a coil made with square or flat wire of rectangular or square cross-section.

In many cases it has been found advantageous to employ a conical or tapering sliding cylinder in connection with a cylindrical coil, or alternately to employ a conically wound coil together with a straight cylinder in order to provide a variable distance between successive winding turns of the coil and said cylinder. In this manner, the capacity between the coil and the sliding cylinder will be a function of the adjusting position of the cylinder, such as in the case of Figs. 5 and 6, and may be varied or controlled to suit any special requirements.

Referring to Fig. 9, there is shown another practical application of a variable tuning coil according to the invention. The circuit shown represents a two-stage push-pull amplifier comprising amplifier tubes 3'! and 38 and 40, 4| and fixed tuning condensers 42, 43 and 44, 45, respectively. The two sliding cylinders I I for each stage are connected with each other both electrically and mechanically and arranged for simultaneous adjustment by means of a common operating member indicated schematically at 46'. The

anode voltage for the tubes of each stage is advantageously applied through the tuning coils, suitable decoupling or by-pass condensers to ground being provided as shown at 46 and 4'1, respectively. The input and output terminals of the circuit are shown at cd and e;, respectively.

Referring to Fig. 10, there is shown a single resonant anode circuit connected to an amplifier tube 48 and comprising a fixed tuning condenser 58 shunted by an adjustable induction coil of the type according to the invention as shown in Fig. 3. Condenser SI of large capacity represents the usual by-pass or decoupling capacity for the anode voltage supplied through the coil ID from a suitable source. In this arrangement, the sliding cylinder I l is substantially free and not in any way directly or conductively connected in the circuit. The high frequency current is transmitted through the relatively large capacity between the sliding cylinder I I and the fixed cylinder I8, the latter being connected to the end of coil I 0 in substantially the same manner as shown and described with reference to Figs. 3 and 4.

Referring to Fig. 11, there are shown a pair of series resonant or trap circuits including adjustable induction coils according to the invention. The inductance of the upper coil ll] of the type according to Fig. 3 together with condenser 52 represents the first series-resonant circuit and similarly the induction of the lower coil ID together with condenser 53 represents the second series-resonant circuit. Both series-resonant circuits are connected, in the example shown, in a symmetrical two-wire line having input terminals (3-11 and output terminals ef, the latter being connected to the fixed cylinders I8 on the one hand and to a load such as a symmetrical cable 56, on the other hand. The two seriesresonant circuits may serve to suppress undesirable harmonics in the currents being transmitted, while freely passing the current of fundamental frequency to which the circuits are tuned. This circuit arrangement also dispenses with any direct circuit connection of the sliding cylinders II, in that the high frequency currents are again directly transmitted through the capacity between the cylinders II and I8, respectively.

While there have been shown and described a few desirable embodiments of the invention, it is understood that this disclosure is for the purpose of illustration and that various changes in shape, proportion, and arrangement of parts, as well as the substitution of equivalent elements for those herein shown and described may be made without departing from the spirit and scope of the invention as defined in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense.

I claim:

1. A variable high frequency inductance comprising a pair of induction coils, a pair of cylindrical non-magnetic metallic members each arranged in spaced and electrically isolated relation to one of said coils, means for electrically connecting said members, whereby to connect the inner ends of said coils adjacent to said members through the mutual capacities between said coils and members, and means for simultaneously axially displacing said members, the outer ends of said coils serving as terminals of said inductance.

2. A variable high frequency inductance comprising a pair of induction coils, a pair of cylindrical non-magnetic metallic members each arranged concentric to and in spaced and electrically isolated relation to one of said coils, the spacing distance between said coils and members being small compared with the distance between adjacent coil winding turns, means for electrically connecting said cylinders, whereby to connect the inner ends of said coils adjacent to said members through the mutual capacities existing between said coils and said members, and means for simultaneously axially displacing said members, the outer ends of said coils serving as terminals for said inductance.

3. A variable high frequency inductance comprising a pair of substantially identical induction coils, a pair of identical non-magnetic metal cylinders each arranged concentric and in spaced and electrically isolated in relation to one of said coils, means for electrically connecting said cylinders, whereby to connect the inner ends of said coils adjacent to said cylinders through the mutual capacities between said coils and cylinders, and means for simultaneously axially displacing said cylinders, the outer ends of said coils serving as terminals of said inductance.

4. A variable high frequency inductance com- 'nrising'apain' oi induction coils, of nonmagnetic hollow-metal; cylinders each arranged concentric-to and overlying one of said coils in spaced and electrically isolated: relation thereto the spacing distance betweensaidcoils and said cylinders being; small compared with the distance; between adjacent coil turns, means V-for electrically connecting said cylinders, whereby to connect, the inner; ends ofsaid coils enclosed byr-saidzcylindersthrough the mutual capacities between the said coils and cylinders and means for: simultaneously axially-displacing said cylinclers; the outer ends of.- said coil-s serving as terminals of said inductancel 5;. Avariablehigh frequency inductance com.- prising, a pair of; substantially identical induction coils; a pair of identical non-magnetic .1101- low metal; cylinders. each arranged concentric to and1 overlying oneof said coils in spaced and electrically isolated relation thereto, the: spacing, distance-betweensaid cylinders and said coils being smallvcompared-with the distance between adjacentwcoi l Winding turns, .means for electrisally aconnectingsaid cylinders, whereby to connect: the inner ends of, said (coils enclosed: by saidcylinders through? the mutual capacities between said coils and (cylinders, and means ,for simultaneously-axially displacing said cylinders, theaouternfreei ends oisaid col-1s serving as terminals of, said inductance;

'62 A, variableghigh frequency inductance comprisinga pair of substantially identical: inductioncoils, a pair of identicalnon-magnetic hollowv metal cylinders .arrangedeaoh' coaxiallywith and overlying equal portions one of said coils in spaced and;electrically-isolated relation thereto, means for directly continuously and mechanically connecting said cylinderswhereby to connect the inner ends of said coils enclosed; by said cylinders through the mutualcapacitieshetween said coils and cylinders, and means for simultaneously axially displacing said cylinders the outer ends-ofasaid coils serving as terminals of said inductance;

GUSTAV GUANELLA.

REFERENCES CITED The following references are of record in the file of this patent:

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